The invention relates to a process for the control of the product composition in an apparatus for sizing and sorting mineral raw materials, in particular sand or gravel, comprising a feed means for the raw material feed and at least one chamber serving for the separation of the low density materials from the sand product fraction, further comprising a sand product removal device and an overflow for low density materials, wherein the chamber is designed as a sorting region according to the fluidized bed process in order to sort the raw material feed. An apparatus in accordance with the aforementioned characteristics for the separation of low density materials from sand and gravel is described in the brochure "MAB Fluidized Bed Sorter-Hydrosort I" of the firm Schauenburg Maschinen-und Anlagen-Bau GmbH; in this apparatus the raw material feed, composed of sand of different particle size, is introduced into the cylindrical single-chamber receptacle comprising a substantially flat bottom and discharge means provided therein for the purified sand. The single chamber receptacle is supplied with upwardly flowing water at the bottom so that in the single chamber receptacle, utilizing the fine fractions of sand contained in the raw material feed, a fluidized bed is generated having a very high turbidity and causing organic impurities, in particular, severely carbonized wood, contained in the raw material feed, to float so that the impurities as well as slurry like micro particles are flushed out with the overflow water.
Additionally, a dual chamber design, comprising an interior chamber serving as a coarse sand chamber for the separation of the coarse sand and an exterior chamber as a fine sand chamber serving for sorting the fine sand according to the fluidized bed process and connected to the coarse sand chamber via an overflow, designed as an inclined surface, and further comprising an overflow for low-density materials associated with the exterior chamber, is described in EP 0 508 335 A2, operating according to the same principle, with regard to the purification of the fine sand, since in a first step the coarse sands may be separated as a product fraction without specific purification due to a set particle size limit.
For apparatus of this type it is now required that the content of micro particle sand in the particular overall sand product mass concerned should not exceed a predetermined percentage and should preferably be in the range of a determined concentration in relation to the particular sand product mass; such a requirement can, however, not be met by operating the afore described apparatus of one or the other embodiment.
It is therefore the object of the invention to provide a process for the control of the product composition in an apparatus having the aforementioned features by which a predetermined concentration of micro particle sand is realized in the sand product mass.